David Jones dusts his house in the Curtis Bay neighborhood of Baltimore almost daily. He rarely opens his windows, even when the weather is beautiful, because the outdoor air makes him feel sick.

Immediately across the street is the Curtis Bay coal terminal, where heaps of coal taller than Jones' house are piled up for overseas shipment. Dust from the coal mounds enters people's cars, homes — and lungs.

"You wake up in the morning and your throat hurts," Jones says. In the bathroom sink, he can see black specks in his spit.

Jones is one of millions of people in the United States who live with dangerous air pollution, including gases like nitrogen oxide and sulfur dioxide, ground-level ozone and tiny particulates that coat every surface. Some particles are so small they worm deep into the lungs and cross into the brain.